The ISC is a technique for defining how to maintain service continuity by using session transfer mechanism in case that a user device UE is moving (since the movement of the UE can not be hided to the IMS session layer, the service continuity can not be maintained). More details about the ISC techniques may refer to 3GPP TS 23.237.
There is an important signaling delay in the ISC session transfer procedure. As known, the ISO session transfer procedure comprises a sub-procedure of remote leg update, which constitutes a main portion of the delay. In an extreme situation, such as roaming, the sub-procedure of remote leg update leads to a great signaling delay, since a lot of nodes are needed in the SIP signaling path to process SIP messages, as shown in FIG. 1. It should be noted that, although as shown in FIG. 1, the remote UE is an IMS terminal, the situation of delay is similar while the remote UE is a PLMN/PST terminal. As shown in FIG. 1, 9 nodes are needed in the IMS signaling path to processing an UPDATE or Re-INVITE message. FIG. 2 shows the end to end session between the local UE and the remote UE before performing the existing ISO procedure (without the optimizing mechanism of the present invention). When the ISO session transfer procedure occurs, a sub-procedure of remote leg update may cause a long delay, as described above.
If the signaling delay is too long, a call will be interrupted while a UE which can perform a dual radio handover between an IP continuity access network (IP-CAN) and a circuit domain network is moving rapidly, and the interruption duration is very long when the UE performs a SRVCC handover from the IP-CAN to the circuit domain network and the service experience degrades, and the call will also be interrupted in the worst situation. Both of the two problems will cause the users' service experience to degrade.